This invention relates to ion thrusters and, more particularly, to the structure and method of fabrication of electron sources used in ion thrusters.
Ion thrusters are used in spacecraft such as communications satellites for stationkeeping and other functions. An important advantage of the ion thruster over an engine using chemical propellants is that it utilizes the electrical power generated by the solar cells of the satellite to achieve the propulsion. The ion thruster requires relatively small amounts of a consumable propellant that is ionized, and it is not necessary to lift large masses of chemical fuel to orbit. The ion thruster also has a high specific impulse, making it an efficient engine which requires very little propellant.
In an ion thruster, a plasma of ions and electrons is created by ionizing a flow of the consumable propellant with electrons emitted by a cathode assembly. The plasma is confined within the body of the ion thruster. Positively charged propellant ions are electrostatically extracted from the plasma and accelerated rearwardly by an ion-optics system. The reaction with the spacecraft drives it forwardly, in the opposite direction. The excess ionic charge due to the ejection of the positive ion beam, which may cause undesirable negative charging of the exterior surface of the spacecraft, is neutralized by the injection of electrons.
Free electrons are generated in the ion thruster at two different locations and for two different purposes. One is in the discharge cathode assembly that generates the electrons used to ionize the injected propellant, thereby forming the plasma. The other is the neutralizer assembly.
Such electron sources generally include an electrical resistance heater that heats a cathode, which in turn emits electrons. In the current technology, the heater is manufactured by swaging a coaxial arrangement consisting of a heater element, its outer sheath, and an insulator that separates them. This coaxial arrangement is coiled on a mandrel to form the heater, which is then assembled over the cathode tube. While operable to produce cathode assemblies, this approach has experienced a low yield of acceptable final articles. The inventors have determined that the low yield is largely due to a lack of controllability of the swaging process. Accordingly, there is a need for a better approach to the structure and fabrication of cathode assemblies for use in ion thrusters. The present invention fulfills this need, and further provides related advantages.
The present invention provides a hollow cathode assembly for use in the discharge chamber or neutralizer of an ion thruster, and a method for its fabrication. The hollow cathode assembly has a physical structure unlike prior structures. The new physical structure allows the use of a fabrication approach which is more controlled than prior, alternative fabrication techniques. The result is an improved hollow cathode assembly, with higher manufacturing yields.
In accordance with the present invention, an ion thruster comprises a hollow cathode assembly. The hollow cathode assembly comprises a heater including an inner ceramic sleeve having an outer surface, and an outer ceramic sleeve having an inner surface. The outer ceramic sleeve overlies the inner ceramic sleeve with a filament volume between the outer surface of the inner ceramic sleeve and the inner surface of the outer ceramic sleeve. The heater further has a wound filament having windings disposed within the filament volume, and a mass of ceramic powder filling a remaining portion of the filament volume between the windings of the filament. A cathode is disposed within the inner ceramic sleeve of the heater. A heat shield desirably overlies the outer ceramic sleeve to improve the efficiency of the heater. The hollow cathode assembly may be used as the electron source in either the discharge cathode assembly (plasma source) or the charge neutralizer, or both.
This new physical structure of the hollow cathode assembly is conducive to the utilization of a well-controlled, highly reliable fabrication procedure. In accordance with this aspect of the invention, a method for making an ion thruster having a hollow cathode assembly includes the preparation of the hollow cathode assembly by the steps of preparing a filament, forming the filament into a cylindrical shape, providing an inner ceramic sleeve, an outer ceramic sleeve, and a mass of ceramic powder, positioning the filament and the mass of ceramic powder between the inner ceramic sleeve and the outer ceramic sleeve, providing a cathode, and positioning the cathode within the interior of the inner ceramic sleeve. The heat shield may be assembled over the exterior of the outer ceramic sleeve. This approach has the advantage that the various components, particularly the filament, may be separately fabricated and inspected, prior to assembly. The assembly does not involve any steps which are difficult to apply reproducibly, such as swaging.
The present approach thus provides a new physical structure and a new fabrication technique for ion thrusters that improve the manufacturing yield. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.